Police found 175 cars unlocked or vulnerable to thieves in a day of action.
Vehicles in the Stairfoot area of Barnsley were checked by officers last week.
Detective Chief Inspector Mark Wilkie said: “The results of this operation were disturbing.
“It seems numerous motorists had not given a second thought to the security of their vehicles while they were shopping.
“Thankfully in this case it was only our officers who were taking a look inside, but next time it could be someone with an entirely different motive.”
Residents are urged to not leave items on display in their cars, remove valuables from their glove box and park in a well lit spot.
Motorcyclists are also advised to keep their machines in a locked garage or shed or chain it to a stationary object.